I hear the argument come up relatively often that the Japanese should have attacked Siberia in WW2, as it would have split Soviet forces and allowed Germany to defeat them in the west.
Non-aggression pact notwithstanding, this never really sounded quite right to me. Outside of Vladivostok, I don't know of any notable cities that far east, the infrastructure would have made advances beyond the coast leave an even heavier burden on the Japanese logistic system already straining to supply their men in China, and if I'm not mistaken, Soviet resources in Siberia weren't being exploited too heavily at the time.
All in all, it seems to me like for such an offensive, Japan would have had to use a lot of men and material to not really accomplish much of note. Is there more to it that I'm missing, or would it have been as ineffective and cost-prohibitive as it seems?
Your pretty much right on the ball there, Japan would not be able to win against the Soviets in the Siberia region. We can look at the undeclared border conflict as a prime example of how Japan would hold up in a war with the Soviet Union. Japan was defeated by Red Army forces in Khalkin Gol pretty badly. The Japanese Army while well equipped and well trained, they were prepared to colonial or garrison troops who weren’t well equipped and relied on the home country to send supplies over the sea. And the Asian troops who ill equipped and poorly trained, and many lacked heavy equipment as well. The Soviet Red Army were better equipped, had a lot of tanks, and modern equipment. This battle showed the IJA that they were not ready at all to fight the Soviets. Not only this the IJA also suffered setbacks in China after Chinese forces counterattack and recovered some lost ground. These two events showed the weakness of the IJA and led to their loss of favor in the Japanese High Command and government. The Japanese Navy now became the clear favorite in the high command and they wanted to strike southward, capitalizing their gains in China and striking the resource rich south. With this plan Japan would rely on her Navy which was one of the best in the world. It’s the smarter move as well, as any counterattack or resupply of troops in the Southeast would require ships and a naval presence in the area. With the European powers in turmoil with the German invasion and the US (hopefully) knocked out, this would give Japan sufficient time to establish control and fortify the area.
You are correct in assuming that it would take time to even extract the resources from Siberia. While it would take less time to extract resources in the Dutch East Indies and British Malaya which already had been extracting resources for years now. The resource problem in Japan reached a crisis mark when the US, UK, and Dutch embargoed Japan and cut off all strategic resources to them. This includes iron, steel, copper, rubber and especially oil. Without oil Japan could not move her ships, supply her troops in the field, or fly her planes. This put Japan in a bad situation as it relied heavily on imports to fuel their war effort. So now when Germany attacked the Soviet Union, Japan had a clear shot to as well from the south and damn their neutrality pact with them. However Japan would run very low on all supplies and would have to also give up some gains, possible all of them in China. As it would need a lot of troops to guard the frontline and invade the Soviet Union. This would give the Chinese a lot of breathing room and now would counterattack and disrupt the Japanese attack.
The Japanese could attack the Soviet Union, but what would they gain from this? Barren land with some promise of resources that will take time to extract when they face completely running out of resources? Japan would have attacked if Russia was in total panic mode and was about to collapse from the German advance. But the Russians never collapsed, the exact opposite happened and they stood firm against the Germans. By 1941 they were fully committed to attacking the Southern area, it could not attack with its army in the North while it was planning a huge operation in the Pacific. The shift towards knocking the US out and taking the Dutch East Indies and British Malaya was the best option for the Japanese to solves its crippling resources shortage.